How do clocks measure time internally

This question sounds simple at first, but it cannot be answered at PTB. As an "eternal riddle", the phenomenon "time" has always occupied philosophers. Augustine confesses: "If nobody asks me about it, I know, but if I want to explain it to someone who asks, I don't know".

"Time is what prevents everything from happening at once!" That is a plausible definition by John A. Wheeler. And already we are with physics and the question of when exactly does something happen and how much time elapses between two events? It is the way physicists uncover inexplicable phenomena by observing them as closely as possible. This is exactly what they do with time: They build clocks that tick incredibly evenly. A leading or trailing edge, as we might know it from our kitchen clock, does not exist with these clocks. Once correctly set, they carefully count off the seconds. Events can be precisely dated and time intervals can be measured. So far, so good, but even the best watch won't tell you why there is time. It just shows that the time is there and passing.


Time and frequency, 4.4

PD Dr. Ekkehard Peik

Telephone: (0531) 592-4400
ekkehard.peik (at)


Birgit Römer

Telephone: (0531) 592-4401
birgit.roemer (at)


Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig